Means for removing protein-bound substances

ABSTRACT

The present invention relates to a means of dialysis for removing protein-bound substances from a biological fluid, especially blood or blood plasma, which contains at least one means for solubilizing protein-binding substances to be removed into the biological fluid and/or dialysis fluid, and to a process for removing protein-bound substances from a biological fluid.

This application is a continuation of international applicationPCT/EP03/04940, filed on May 12, 2003.

The present invention relates to a means for removing protein-boundsubstances from a biological fluid, especially blood or blood plasma,which contains at least one means for changing the concentration ratioof toxin-protein complex to free toxin and free protein in thebiological fluid and/or dialysis fluid, and to a process for removingprotein-bound substances from a biological fluid. The term “toxin” isunderstood very broadly here and additionally covers all protein-boundsubstances which normally are not directly referred to as toxins, suchas drugs, electrolytes, hormones, fats, vitamins, gases, and metabolicdegradation products like bilirubin.

In a number of diseases, particularly including acute or chronic kidneyfailure, acute or chronic liver failure or exogenous intoxications,pathogenic substances dissolved in the plasma or bound to proteins mustbe removed from the blood. The conventional processes of haemodialysis,haemofiltration or haemodiafiltration only allow a small proportion ofprotein-bound substances to be eliminated.

The deficient elimination of protein-bound substances is currently onlyof secondary importance in renal function replacement.

The life expectancy of patients with acute or chronic kidney failure canbe normalized by a renal replacement therapy (with recovery of the organfunction in the case of acute kidney failure) or at least prolonged fora number of years (in the case of patients still requiring a renalreplacement therapy).

The situation is quite different for acute liver failure or an acuteworsening of chronic liver failure. Methods comparable to renalreplacement therapy are currently not available in this case.

The hepatic function can essentially be subdivided into two mainfunctions:

-   -   the synthesis of vital proteins and    -   the removal of mainly protein-bound toxins.

Basically only liver transplantation is currently available to replacethe synthetic function. Although so-called bioreactors are known, withcells that at least partially take over the synthetic function of normalliver cells, these can only be used experimentally at the present timeand their function is still insufficient. Liver transplantations areperformed on approx. 20% of patients with acute liver failure because noadequate process for taking over the detoxication function exists, sothe time taken for the hepatic function to recover cannot be bridged.

Protein-bound substances probably play an important role in thepathogenesis of hepatic encephalopathy, hepatic pruritus and hepatorenalsyndrome. These pathogenic substances, which are bound predominantly toalbumin, include especially aromatic compounds like phenol derivatives,indole derivatives, furan derivatives or aromatic amino acids,bilirubin, C₄-C₇ carboxylic acids, mercaptans, substances similar todigitoxin and benzodiazepine, and metal cations like copper cations,aluminium cations or iron cations. One of the most important diseaseshere is hepatic encephalopathy as it can be life-threatening and/orleave permanent damage.

Since the 70's there have been a variety of attempts, based largely onthe dialysis technique, to replace the detoxication function of theliver:

1. Haemodialysis, Haemofiltration or Haemodiafiltration

These conventional processes are currently used on many patients withliver failure because in most cases, in the final stage of liverdisease, a hepatorenal syndrome occurs which also leads to kidneyfailure that has to be treated by dialysis. However, these processes donot achieve a sufficient removal of protein-bound substances andessentially only eliminate water-soluble substances of low orintermediate molecular weight.

2. Haemoperfusion

In this process, blood or plasma is passed over an adsorber (charcoaland/or cation or anion exchanger) in order to remove the protein-boundtoxins (O'Grady J. G., Gimson A. E., O'Brien C. J., Pucknell A., HughesR. D., Williams R.: Controlled trials of charcoal hemoperfusion andprognostic factors in fulminant hepatic failure. Gastroenterology, 94,1186-1192, 1988). This method has the disadvantage of beingnon-specific, so vital substances are also removed from the blood orplasma.

3. Albumin as Adsorber with Two Different Processes

A. MARS (Molecular Adsorbents Recirculating System)

The MARS described in EP 0 615 780 uses a special albumin-coateddialysis membrane. The recirculating albumin-containing dialysate ispassed over 2 adsorber columns (charcoal and resin) in order toeliminate the protein-bound toxins removed from the patient by dialysisand to prepare the binding sites of the albumin in the dialysate fortoxins (Stange J., Hassanein T. I., Mehta R., Mitzner S. R., Bartlett R.H.: The molecular adsorbents recycling system as a liver support systembased on albumin dialysis: a summary of preclinical investigations,prospective, randomized, controlled clinical trial, and clinicalexperience from 19 centers. Artif. Organs, 26, 103-110, 2002).

B. Albumin Dialysis

Albumin dialysis is a process related to continuous haemodialysis. Afeature of continuous renal replacement therapy is the use of slowdialysate flows (1-2 l/h compared with 30 l/h in normal dialysis). Inalbumin dialysis, in contrast to conventional continuous renalreplacement therapy, albumin is added to the dialysate to give a 5%solution (Kreymann B., Seige M., Schweigart U., Kopp K. F., Classen M.:Albumin dialysis: effective removal of copper in a patient withfulminant Wilson disease and successful bridging to livertransplantation: a new possibility for the elimination of protein-boundtoxins. J. Hepatol., 31, 1080-1085, 1999). The use of albumin is basedon its being the main carrier protein for protein-bound toxins in theblood.

For use on large numbers of patients, all the albumin-based systemsdescribed above incur high treatment costs (current treatment costs forMARS or albumin dialysis are approx. 2500 euros/day). Furthermore, thesesystems offer an unsatisfactory detoxication efficiency: on average onlyan approx. 10-30% reduction in the bilirubin level as a marker forprotein-bound substances. Although the albumin-based dialysis processesbring about an improvement in the symptoms of hepatic encephalopathy, anormalization of the values cannot be achieved as a consequence of thehigh treatment costs.

The object of the present invention is therefore to provide a means ofdialysis and a dialysis process with which protein-bound substances,together with other unwanted dialysable substances, can be removed froma biological fluid in one step in a simple and cost-effective manner.

This object is achieved according to the invention by a means accordingto claim 1 and a process according to claims 14 and 15.

The object is achieved according to the invention by a process in which,with a suitable means, the concentration ratio of toxin-protein complexto free toxin and free protein in a dialysis fluid circuit, in abiological fluid circuit or in both circuits is shifted in favour of thefree substances, and the toxin is then removed. Analogously it is alsopossible to consider the concentration ratio of adsorbed toxin to freetoxin and adsorbent.

This solution has the advantage that, because they have beensolubilized, the substances can be removed easily and efficiently fromthe biological fluid.

Within the framework of the present invention, dissolved substances andfree toxin are understood as meaning not only individual moleculessolvated by the solvent, but also those bound to dialysable substances.Toxins bound to dialysable substances can also be dialysable ascomplexes.

This process not only affords a rapid dialysis, coupled with anappreciable cost advantage, but also makes it possible to achieve aparticularly thorough purification of the biological fluid.

In the case of blood or blood plasma purification, this process furtheraffords a more patient-friendly method of treatment which, in the areaof emergency medicine, for acutely life-threatening conditions, can beof crucial importance for the success of the treatment.

The means for changing the concentration ratio of toxin-protein complexto free toxin and free protein comprises a device for adjusting the pHof the usable fluids, a device for adjusting the temperature of theusable fluids, a device for adding substituate to dilute or change thecomposition (for instance the salt content) of the usable fluids, adevice for adding dialysable compounds binding to the substances to beremoved, and a device for irradiating the usable fluids with waves.Within the framework of the present invention, the different devices canbe combined with one another in any desired manner. It is preferable touse at least one device for adjusting the pH and at least one device foradjusting the temperature in a circulation system.

One advantage of the present invention is that, with simple means, i.e.conventional devices for adding solutions of acids, bases, substituateor dialysable substances, or conventional heating, cooling or ultrasonicapparatuses or other generators of light, infrared, ultraviolet orelectromagnetic waves (hereafter referred to as waves), the substancesto be removed can be solubilized in a simple and cost-effective mannerby weakening the bond between the protein-binding substances and carrierproteins or adsorbers. The dissolved substances (toxins) are dialysableand hence easy to remove.

It has in fact been found that the binding affinity of carrier proteinslike albumin or adsorbers for toxins can be selectively lowered by anumber of measures, thereby increasing the concentration of the freetoxins in the solution. The protein-bound substances in the biologicalfluid or dialysis fluid are actually in equilibrium with a small amountof non-bound substances. Lowering the binding affinity makes it possibleto increase the concentration of the non-bound, dialysable substances,the free substances passing into solution.

These measures include raising the temperature of the fluid containingbound toxins, changing the pH (acidic and basic range), irradiating withwaves, diluting the fluid, changing the salt content and introducingdialysable substances that bind to bind to the toxins. The last of thesemeasures results in the formation of dialysable complexes.

If provision is made in the dialysis fluid circuit for at least onemeans according to the invention for changing the concentration ratio oftoxin-protein complex to free toxin and free protein, the dialysis fluidwhich can be used in the means of dialysis should contain an adsorberfor the substances to be removed from the biological fluid. A smallproportion of the protein-binding toxins in the biological fluid is inthe free form in solution and this proportion can diffuse through thesemipermeable membrane in the dialyser and bind to the free bindingsites of the adsorber in the dialysis fluid. Then, via a means accordingto the invention for changing the concentration ratio of toxin-proteincomplex to free toxin and free protein, for example a device for addingacid, the binding affinity between adsorber and toxin is at leasttemporarily lowered, whereby the substances to be removed pass intosolution and can be removed from the dialysis circuit via dialysis(diffusion) or filtration (convection) or a combination of bothprocesses, hereafter called diafiltration.

According to the invention, adsorber and free toxin can also beseparated by centrifugation, as used e.g. in plasma separation forcarrying out plasmapheresis. Within the framework of the presentinvention, centrifugation thus constitutes an alternative to dialysis asa separation method.

Provision for at least one means according to the invention for changingthe concentration ratio of toxin-protein complex to free toxin and freeprotein can also be made in the circuit of the biological fluidcontaining substances to be removed, bound to a carrier protein. Thus,for example, the effect of increasing the temperature is to lower thebinding affinity of the carrier protein and solubilize the substances,enabling the dissolved, free substances to be dialysed.

Preferably, both the biological fluid circuit and the dialysis fluidcircuit contain at least one means according to the invention forchanging the concentration ratio of toxin-protein complex to free toxinand free protein.

One advantage of the invention is that substances with different bindingbehaviour can be removed by the various means available, especially ifdifferent measures are combined with one another.

One particularly efficient and cost-effective means is the device foradjusting the pH of the fluids. Adjusting the pH of the fluids to theacidic and/or basic range makes it possible selectively to influence thebinding of different substances to the carrier proteins or adsorbers.Thus, adding an acid makes it possible to lower the pH of the fluid,thereby reducing the binding of certain toxins to proteins in the acidicrange and hence increasing the concentration of free toxins in thefluid.

For example, the binding of copper ions to albumin can be weakened inthis way so that free, dissolved copper ions in the following filter canbe removed by dialysis, filtration, diafiltration or centrifugation.Analogously, the pH of the fluid can be adjusted to the basic range sothat the toxins liberated in the alkaline range can then be eliminatedfrom the fluid by dialysis, filtration or diafiltration.

Analogously, by adding a base, it is possible to weaken the binding ofcertain toxins to proteins and hence increase the concentration of freetoxin in the fluid, a pH range of 8-13 being preferred in this case.

After removal of the toxins from the fluid by means of dialysis,filtration, diafiltration or centrifugation, the pH is optionallyadjusted to a different advantageous value.

This can be of advantage particularly when working with an adsorber,e.g. albumin. A different advantageous pH is then chosen so that theaffinity of the adsorber for the toxin is increased again. This allowsrecycling of the adsorber and hence offers a decisive cost advantage.

A circulation system (for dialysis fluid or biological fluid to bepurified) contains preferably two and very particularly preferably threedevices for adjusting the pH, so that the pH can be adjusted to theacidic or basic range with the first device, to the basic or acidicrange, respectively, with the second device and back to the originalrange (usually neutral) with the third device. As a further preference,a circulation system (for dialysis fluid or biological fluid to bepurified) contains two devices for adjusting the temperature so that theusable fluid can for example be heated and then brought back to theprevious temperature, or to another desired temperature, by cooling.Very particularly preferably, a circulation system contains threedevices for adjusting the pH and two devices for adjusting thetemperature.

Another advantage of the invention is that, by means of dialysis,filtration or diafiltration devices provided in the dialysis fluidcircuit and/or biological fluid circuit, the dissolved, dialysablesubstances can be removed from the fluids (dialysis fluid or biologicalfluid to be purified) easily and efficiently after separation from thecarrier proteins or adsorbers. This can be done using conventionaldialysis apparatuses such as the ones known to those skilled in the art.In addition, it is preferable to use devices for changing thepH/temperature values and devices for appropriate monitoring of thesechanges. Advantageously, in the means according to the invention, adialysis, filtration, diafiltration or centrifugation device is inserteddownstream from a device for adjusting the pH/temperature of the fluidto be used, in order to remove the free, dissolved substances directlyfrom the fluid. A particularly preferred means is one in which a devicefor adding acid or base, a dialysis, diafiltration, filtration orcentrifugation device, a device for adding base or acid, a dialysis,filtration, diafiltration or centrifugation device and a device foradding acid or base are provided, in that order, in the dialysis fluidcircuit and/or biological fluid circuit. This makes it possible toremove different protein-binding substances from the dialysis fluid andbiological fluid very efficiently and the purified dialysis fluid can inturn be recycled to the dialyser to recharge the adsorber withprotein-binding substances.

An advantage of one particular embodiment of the present invention,where a means for changing the concentration ratio of toxin-proteincomplex to free toxin and free protein, for example a device foradjusting the pH, is arranged only in the biological fluid circuit, isthat, to remove unwanted protein-bound toxins, the dialysis fluid doesnot necessarily have to contain an adsorber, for example the acceptorprotein albumin, which drastically reduces the dialysis costs.

Devices for adjusting the pH include especially devices for adding acidor base, for example metering pumps. Appropriate acids or bases areaqueous solutions of biologically compatible acids or bases. It isgenerally preferable to use acids or bases whose conjugate bases oracids are ions that occur naturally in the human organism. Examples ofacids which can be used are hydrochloric acid, sulfuric acid or aceticacid, hydrochloric acid being preferred. Examples of bases which can beused are sodium hydroxide solution or potassium hydroxide solution,sodium hydroxide solution being preferred. The biological or dialysisfluid can be adjusted e.g. to a pH of between 1 and 7, advantageously ofbetween 2.5 and 5, by adding acid, and to a pH of between 7 and 13,advantageously of between 8 and 13, by adding base. In each particularcase the desired pH depends substantially on the nature of the fluidused, the nature of the protein and the properties of the substances tobe removed. For example, the binding affinity of copper for albumin issignificantly lowered in the pH range around two. Conversely, this meansthat copper has a particularly high binding affinity for albumin at a pHabove about 3. It has also been observed, for example, that the bindingaffinity of bilirubin for albumin is significantly lowered at a pH ofabout 12.

Devices for adjusting the temperature include especially heating devicessuch as conventional heating apparatuses, microwave apparatuses orinfrared apparatuses, or cooling devices such as conventional coolingunits. One or more heating/cooling devices can be arranged in thedialysis fluid circuit and/or biological fluid circuit. In particular,the substances to be removed can be solubilized by heating or coolingthe usable fluids, while the biological fluid or dialysis fluid can bebrought back to the desired temperature by cooling or heating. Thenature and extent of the temperature gradient used depends on the natureof the fluid, the adsorber and the toxin to be removed. For example, itis possible to heat first and then cool again. The reverse process mayalso be advantageous. It may also be advantageous to carry out theheating/cooling stepwise.

Another advantage of the invention is that the binding affinity of theadsorber can be selectively increased by a device for cooling or heatingthe usable dialysis fluid, whereby free, dissolved substances that havediffused into the dialysis fluid can be bound by recycled adsorbers.

The desired temperature of the fluids to be used is substantiallydependent on their nature. If the biological fluid used is blood orpartial blood products like blood plasma or fractions thereof, it ispossible to heat to a temperature of up to approx. 150° C. (coupled witha corresponding pressure increase, e.g. as used in the heatsterilization of milk), preferably of up to 45° C. Thus heating beyondthe physiological range is also possible. When using the means ofdialysis according to the invention in an extracorporeal circuit on apatient, the temperature can be lowered again to an optimum value forthe patient in the range from 35 to 37° C., or approx. 35° Celsius inthe case of patients with hepatic encephalopathy.

If the device for adjusting the temperature is used in the dialysisfluid circuit, the temperature can also be increased to over 150° C.according to the addition e.g. of steam, or a pressure increase, orother stabilizers (known from the pasteurization treatment of albumin).

The heating of the fluid to be used, in the circulation system, can beeffected via direct heating of the fluid-filled tubing system by meansof a heating apparatus or by means of irradiation with microwaves orinfrared. It may be sufficient e.g. to have heating devices only in thedialysis fluid circuit, the biological fluid nevertheless being heatedas well because of the heat exchange between the fluid to be purifiedand the dialysis fluid in the dialyser. In another embodiment, upstreamfrom the entrance to the dialysis fluid circuit or biological fluidcircuit, a heating device can also be inserted downstream from a devicefor adjusting the pH or a device for adding substituate. In this casethe dialysis fluid and/or the fluid to be purified are heated by addingwarm solutions.

An ultrasonic apparatus, for example, can be used as the device forirradiating with waves. Other appropriate devices are those suitable forgenerating light waves, ultraviolet waves, infrared waves, radio wavesand microwaves.

Another possible means for changing the concentration ratio oftoxin-protein complex to free toxin and free protein is a device foradding dialysable compounds binding to the substances to be removed.Said means can be conventional metering pumps which introduce aqueoussolutions of the dialysable compounds. The dialysable substances, someof which are bound to toxins, can easily be removed via the conventionaldialysis or diafiltration devices. Binding compounds which can be usedare dialysable compounds of low/intermediate molecular weight that aredistinguished by a strong affinity for the substances to be removed. Thepreferred compounds include caffeine, which binds to bilirubin, andcommon chelating agents like penicillamine, trientine, deferoxamine,preferiprone, HBED, vitamin C, BAL, DMPS or DMSA, which bind to metalcations such as copper ions or iron ions. The dialysable compounds canbe added both to the biological fluid and to the dialysis fluid, butpreferably to the dialysis fluid in order to avoid complications due topossible contamination of the biological fluid in the event ofincomplete removal by dialysis.

Furthermore, synergistic effects can also occur when using two means forchanging the concentration ratio of toxin-protein complex to free toxinand free protein, for example an increase in pH and the addition of abinding compound (e.g. caffeine).

Devices for adding substituate to dilute or change the salt content ofthe usable fluids include conventional metering pumps with which asubstituate solution can be added. Such a device is preferably used incombination with a heating device, inserted downstream therefrom, sothat warm substituate is added to the circuit of the fluids used.Suitable substituate solutions are aqueous solutions which can containvarious salts as well as urea. These solutions can be commercialdialysis fluids which, as required, can be adjusted to the desiredconcentration by adding salts. However, as mentioned above, it is alsopossible to use stabilizers, agents for thinning the blood, such asheparin or citrate, or substances for changing the osmotic equilibrium,such as salts, or for changing the electrophysiological equilibrium(Donnan effect), such as negatively or positively charged substances.The substituate serves not only to solubilize the substances to beremoved by changing the salt concentration in the fluid. The saltconcentration of the biological fluid, e.g. blood, can also be preciselyadjusted according to the patient's condition by adding substituate.Moreover, it can also be used to restore the binding capacity ofrecycled adsorber for toxins in the dialysis circuit. Also, the additionof urea may be necessary to improve the binding capacity of theadsorber.

The dialysers used can be conventional dialysers currently used e.g. forhaemodialysis. It is also conceivable, however, to consider membraneswith larger pores than those presently used for dialysis. The dialyseris equipped with a conventional semipermeable dialysis membrane; thediffusion through the membrane can optionally be supported by convectivetransport by means of filtration. The dialyser essentially comprises twochambers separated by a dialysis membrane, to each of which is connecteda circulation system (tubing system) for the fluids to be used. Thebiological fluid to be purified and the dialysis fluid areconventionally conveyed in counter-current, but they can also beconveyed in co-current. Conventional components of a dialyser, such asmanometers, air detectors, pumping devices like heparin pumps, bloodpumps, etc., form part of the means according to the invention. Themeans according to the invention can achieve both slow dialysate flows(1-2 l/h) and normal dialysate flows (25-50 l/h) as well as intermediaterates, as required.

The biological fluids which can be used in the means according to theinvention or in the process according to the invention include all humanor animal body fluids, especially blood or blood plasma, particularlypreferably of human origin. The removal of protein-bound substances fromthe biological fluids used is simultaneously accompanied by theelimination of water-soluble substances, for example urea or variousions, that can normally be removed in conventional dialysis. Theprotein-binding substances to be removed are preferably bound to thecarrier protein albumin. The means according to the invention isparticularly suitable for purifying blood and plasma in the medicalsector and can be used both in the field of blood bank processing andfor extracorporeal dialysis on patients.

The dialysis fluids used can be conventional dialysis fluids such as theones known to those skilled in the art. The ion concentration can beadapted to the individual patient's needs. Customary ion-containingaqueous solutions or pure water can be used, as required. Theconventional dialysis fluids are optionally provided with an adsorberfor the protein-binding substances to be removed. Examples of possibleadsorbers are resinate and acceptor proteins. A preferred acceptorprotein is albumin, which can be human serum albumin, animal albumin orgenetically engineered albumin. It is particularly preferable to usehuman serum albumin. The serum albumin solutions can optionally bediluted with water, conventional dialysis fluids or other fluids. Thedialysis fluid used can contain human serum albumin in a concentrationof 0.1 to 25 g per 100 ml, preferably of 2 to 10 g per 100 ml andparticularly preferably of 4 to 6 g per 100 ml.

Other dialysis fluids which can be used are blood, blood serum or freshfrozen plasma. The dialysis fluid can also be dialysate from thebioreactor. Enormous quantities of blood are currently needed forbioreactors (systems working with living liver cells for hepaticreplacement therapy); thus, up to one litre of blood has to be withdrawnfrom the patient's circulation during the circulation of the bioreactor.To stimulate the synthetic function of the liver cells in thebioreactor, however, it may also be sufficient to employ a system whichuses a dialysate containing toxic substances that are normally removedin the liver. Accordingly, a dialysate as described under 1 and 2 canfirst be passed through the bioreactor in the extracorporeal circuit.This dialysate is then purified as described under 2 and the dialysatereturned to the patient. To do this it may be necessary to add albumincontinuously to the dialysate or to use a capillary or a membrane thatis more permeable to albumin than dialysis filters used at the presenttime.

The means according to the invention can be equipped with one or moreconventional pH meters and/or thermometers for monitoring thecorresponding properties of the fluids used.

The invention further relates to a process for removing unwantedsubstances from a biological fluid, comprising the dialysis of abiological fluid against a dialysis fluid through a semipermeablemembrane, characterized in that the dialysis fluid contains an adsorberfor protein-binding substances to be removed, and in that the dialysisfluid is adjusted, by adding acid, base or dialysable substances, bydilution, by changing the salt content, by irradiation with waves or byheating, in such a way that the binding affinity of the adsorber for thebound substances is at least temporarily lowered, thereby increasing theconcentration of the free unwanted substances in the dialysis fluid.

Provision is also made for a process for removing unwanted substancesfrom a biological fluid, comprising the dialysis of a biological fluidagainst a dialysis fluid through a semipermeable membrane, characterizedin that the biological fluid is adjusted, by adding acid, base ordialysable substances, by dilution, by changing the salt content, byirradiation with waves or by heating, in such a way that the bindingaffinity of the carrier protein for the bound substances to be removedis lowered, thereby increasing the concentration of the free unwantedsubstances in the biological fluid.

Also advantageous is a process using a circulation system whichcomprises an at least two-fold addition of acid, base or dialysablesubstances, dilution, changing of the salt content, irradiation withwaves or heating/cooling of the dialysis fluid or biological fluid.

The processes and means according to the invention can be used ingeneral for purifying biological fluids. Biological fluids include allhuman or animal body fluids, especially blood or blood plasma,particularly preferably of human origin. It is possible here to returnthe withdrawn fluids, especially blood or blood plasma, to the body ormake them available for other purposes. Thus, for example, blood bottlescan be purified or the purified biological fluids are made available forother commercial purposes or research purposes.

The means described above is suitable for carrying out the processesaccording to the invention. Further details, features and advantages ofthe process can be found in the above discussion of the means and in theclaims.

Three Examples of the invention are illustrated in greater detail belowwith reference to the Figures. The Figures are diagrammaticrepresentations of particular embodiments of the means according to theinvention.

FIG. 1 is a simplified diagrammatic representation of an embodiment ofthe means according to the invention with heating and cooling devicesand a device for adding substituate in the extracorporeal circuit.

FIG. 2 is a simplified diagrammatic representation of an embodiment ofthe means according to the invention with devices for adjusting the pHin the extracorporeal circuit.

FIG. 3 is a simplified diagrammatic representation of an embodiment ofthe means according to the invention with heating and cooling devices,devices for adjusting the pH and a device for adding substituate in thedialysis fluid circuit.

FIG. 1 shows a means of haemodialysis consisting essentially of adialyser (1), a dialysis fluid circuit (2) (only suggested in theFigure: used dialysate does not have to be recycled in this embodiment),a blood circuit (3) (only suggested in the Figure), heating and coolingapparatuses (6), a device (7) for adding substituate and a thermometer(10).

Via the device (7), substituate, e.g. from conventional haemofiltrationsolution, heated in the heating apparatus (6), is added to the blood inthe blood circuit (3) before it enters the dialyser (1). The warm bloodthen enters the blood chamber of the dialyser (1). Because thetemperature of the blood has been raised, there is an increasedliberation of protein-bound substances from the carrier proteins, thusproducing an increased pool of dissolved, dialysable toxins whichdiffuse through the dialysis membrane into the dialysis chamber of thedialyser (1). When the blood, purified of protein-bound substances, hasleft the dialyser (1), it is cooled again by the cooling unit (6) to aphysiologically acceptable temperature, which is checked by thethermometer (10). Alternatively, the blood temperature and hence thepatient's temperature can also be adjusted by controlling the dialysatetemperature. The blood is then returned to the blood circuit (3).

FIG. 2 shows a means of haemodialysis consisting essentially of adialyser (1), a dialysis fluid circuit (2) (only suggested in theFigure: used dialysate does not have to be recycled in this embodiment),a blood circuit (3) (only suggested in the Figure), metering pumps (4)for adding acid or base, a dialyser (5) and a pH meter (11). By means ofthe metering pump (4), HCl solution is added to the blood in the bloodcircuit (3) before it enters the dialyser (1). This lowers the pH of theblood and some of the toxins pass into solution. The acidified bloodthen enters the blood chamber of the dialyser (1). The dissolved,dialysable substances can diffuse through the dialysis membrane into thedialysis chamber of the dialyser (1). When the blood, partially freed ofprotein-bound substances, has left the dialyser (1), NaOH solution isadded by means of the metering pump (4), whereby the pH is adjusted tothe basic range and further protein-binding toxins pass into solution.Downstream the blood enters another dialyser (5), where anotherdialysis, filtration or diafiltration is carried out in order toeliminate otherwise protein-bound substances dissolved in the alkalinerange. The pH is then adjusted to approx. 7.4 in the neutral range withHCl solution via a metering pump (4), this being checked by the pH meter(11). The blood is then returned to the blood circuit (3).

FIG. 3 shows a means of haemodialysis consisting essentially of adialyser (1), a dialysis fluid circuit (2), a blood circuit (3) (onlysuggested in the Figure), metering pumps (4) for adding acid or base,dialysers (5), heating and cooling apparatuses (6), a device (7) foradding substituate, a device (8) for adding caffeine, a pH meter (11)and a thermometer (10).

Via the device (7), substituate, e.g. from haemofiltration solution,heated in the heating apparatus (6), is added to the blood in the bloodcircuit (3) before it enters the dialyser (1). The warm blood thenenters the blood chamber of the dialyser (1). Because the temperature ofthe blood has been raised, there is an increased pool of free,dialysable toxins which diffuse through the dialysis membrane into thedialysis chamber of the dialyser (1). The dialysis fluid also containsalbumin, which binds to the toxins, so the pool of free substances inthe dialysis fluid is kept low, thereby enhancing the diffusion of thetoxins into the dialysis fluid. When the blood, purified ofprotein-bound substances, has left the dialyser (1), it is returned tothe blood circuit (3).

The dialysis fluid from the dialyser (1), containing albumin-boundtoxins, enters the dialysis fluid circuit (2). HCl solution is added tothe dialysis fluid via the metering pump (4). This lowers the pH of thedialysis fluid and the pool of dissolved, free toxins in the fluidincreases. Arranged downstream in the dialysis fluid circuit (2) is aheating apparatus (6) which heats the dialysis fluid to 41-45° C.,whereby the pool of free toxins is increased further and the proportionof protein-bound toxins falls. The next component in the circulationsystem (2) is a caffeine metering pump (8). The addition of caffeinebinds bilirubin in particular, thereby reducing the proportion ofprotein-bound bilirubin in the dialysis fluid. Downstream the dialysisfluid enters a dialyser (5), where some of the dialysis fluid iswithdrawn from the system in order to keep the concentration of theadsorber in the desired range. In addition, the dialysate is purified bydialysis, filtration or diafiltration, especially to remove free,protein-binding substances and caffeine-bound bilirubin. The albumincannot pass through the filter due to its high molecular weight.Arranged downstream from the exit from the dialyser (5) in the dialysisfluid circuit (2) is a metering pump (4) for adding NaOH solution, aheating apparatus (6) being arranged upstream from the entrance to thecircuit. Downstream there follows another dialyser (5), which withdrawsthe added fluid from the system and eliminates substances dissolved inthe alkaline range by dialysis, filtration or diafiltration. The nextcomponent in the circulation system (2) is a cooling device (6) by meansof which the temperature of the dialysis fluid can be adapted accordingto the desired temperature of the patient. The following metering pump(4) is used to add HCl solution to the dialysis fluid in order to adjustits pH to the neutral range, so the binding capacity of albumin isincreased again and the pH of the blood does not have an adverseinfluence in the dialyser. The next components in the circulation system(2) are a pH meter (11) and a thermometer (10) for checking the pH andtemperature of the purified dialysis fluid before it reenters thedialyser (1).

1. An apparatus for dialysis of a biological fluid containing at leastone protein-binding substance to be removed, comprising: a biologicalfluid circuit containing a dialyser, a dialysis fluid circuit containingat least one second dialyser and a means for at least partiallysolubilizing the protein binding substances to be removed in thedialysis fluid, wherein the dialysis fluid in the dialysis fluid circuitcontains an adsorber for the substances to be removed from thebiological fluid, and wherein the means for at least partiallysolubilizing the protein binding substances to be removed comprises adevice for adjusting the pH of the dialysis fluid to 8-13 downstream ofthe second dialyser and a device for adjusting the pH of the dialysisfluid to 2.5-5.5 upstream of the second dialyser, or a device foradjusting the pH of the dialysis fluid to 8-13 upstream of the seconddialyser and a device for adjusting the pH of the dialysis fluid to2.5-5.5 downstream of the second dialyser.
 2. The apparatus according toclaim 1, wherein the device for adjusting the pH of the dialysis fluidcomprises a device for adding base and a device for adding acid.
 3. Theapparatus according to claim 1, further comprising a heating device or acooling device.
 4. The apparatus according to claim 3, wherein theheating device comprises a heating apparatus and a microwave apparatusor an infrared apparatus, and the cooling device is a cooling unit. 5.The apparatus according to claim 1 further comprising at least oneadditional dialyser in the dialysis fluid circuit.
 6. The apparatusaccording to claim 3 wherein the heating device allows for heating thedialysis fluid to 41-45° C.
 7. The apparatus according to claim 1,wherein the adsorber comprises albumin.
 8. The apparatus according toclaim 1, wherein the dialysis fluid contains human serum albumin in aconcentration of 1 to 25 g per 100 ml.
 9. The apparatus according toclaim 1 further comprising a device for adding a substituate to diluteor change the salt content of the dialysis fluid.
 10. The apparatusaccording to claim 1, wherein the apparatus is adapted to recirculatethe dialysis fluid.
 11. The apparatus according to claim 1 wherein thesecond dialyser is a filter.
 12. The apparatus according to claim 1,wherein the device for adjusting the pH of the dialysis fluid to 8-13 isdownstream of the second dialyser and the device for adjusting the pH ofthe dialysis fluid to 2.5-5.5 is upstream of the second dialyser.